ABSTRACT (Environmental Science and Engineering) Research Project 4 Project 4 is one of two Environmental Science and Engineering (ESE) Research Projects for the proposed ?Center for Environmental and Health Effects of PFAS? being led by North Carolina State University (NC State). The primary goal of the proposed Center is to provide highly relevant data and information to help the Superfund Research Program (SRP) address the growing problem of per- and polyfluoroalkyl substance (PFAS) contamination across the US, including contamination of drinking water sources. PFAS are considered contaminants of emerging concern, and remediation of PFAS-impacted sites is a critical and timely public health challenge. Granular activated carbon (GAC) adsorption is the most widely employed PFAS remediation technology. Although much is known about sorption of organic contaminants by GAC, predicting GAC effectiveness from laboratory data or from fundamental pollutant and GAC properties remains a significant challenge. Our long-term objective is to develop models that predict sorption of organic micropollutants, including PFAS, in GAC treatment systems. A critical barrier to improving existing models is that accessibility of sorption sites inside of GAC particles is not known. An important assumption of current models is that contaminants are uniformly distributed inside of GAC particles at sorption equilibrium. However, direct observations of sorbed contaminants suggest that sorption can occur preferentially near the external sorbent surface. This distinction is significant because it can explain why PFAS sorption capacity increases with decreasing GAC particle size and why laboratory experiments overestimate PFAS removal effectiveness of GAC. Our overarching hypothesis is, therefore, that sorption of PFAS (as well as many other organic pollutants) occurs preferentially in a shell region near the external GAC surface. The shell adsorption hypothesis will be evaluated by (Aim 1) observing and describing intraparticle PFAS distributions at sorption equilibrium and (Aim 2) quantifying and describing PFAS adsorption/desorption kinetics. Using innovative approaches, such as isotope microscopy, we will begin to open the ?black box? that GAC still represents and directly observe intraparticle PFAS distributions. We will use information from direct observations in conjunction with results from sorption equilibrium and kinetic experiments to explain our data with a shell adsorption model. We expect that model parameters will be physically meaningful and can be predicted from fundamental sorbent and sorbate properties. Results of this project will support the development of effective sorbents for PFAS removal, the design of (cost-)effective GAC treatment systems for PFAS remediation, and the evaluation of management options for spent GAC.